Anti-skid brake control system with operational mode control and method therefor

ABSTRACT

An anti-skid brake control system controls the operation mode of a pressure control valve in a hydraulic brake circuit for increasing the fluid pressure in its application mode, for decreasing the fluid pressure in its releasing mode and for holding the fluid pressure at a constant value in its holding mode. The control system has a predetermined operation pattern in which the operation modes of the pressure control valve cycle through a predetermined schedule. Measured wheel acceleration and slip rate values dictate which operation modes are to be selected next and when the change from mode to mode should be performed. When the operation mode is to be changed, the next operation mode selected is checked to see if it is consistent with that scheduled. When the selected operation mode is different from the scheduled one, the selected operation mode is modified to perform the operation according to the scheduled operation mode which would otherwise be skipped.

BACKGROUND OF THE INVENTION

The present invention relates generally to an anti-skid brake controlsystem for controlling a hydraulic automotive brake system to increase,decrease or hold constant the working fluid pressure in a wheel cylinderin the brake system. More particularly, the invention relates to ananti-skid brake control system in which the operation mode is determinedon the basis of wheel speed and wheel acceleration and varies inaccordance with a predetermined pattern in relation to the wheel speedand the wheel acceleration.

In general, wheel slippage during braking is best maintained in apredetermined range so that skidding or locking of the wheels can bereliably prevented and that the braking characteristics of the vehiclecan be optimized. As is well known, in anti-skid control, the brakingpressure to be applied to wheel cylinders is to be so adjusted that theperipheral speed of the wheels during braking is held to a given range,e.g. 80%, of the vehicle speed. Such practice has been believed to beeffective especially when road conditions and other factors are takeninto consideration. Through out the accompanied disclosure, the ratio ofwheel peripheral speed to vehicle speed will be referred to as "sliprate" or "slip ratio".

In the anti-skid control, working fluid pressure in a wheel cylinder ina hydraulic brake circuit is associated with a pressure control valvewhich is operative in an application mode for increasing the fluidpressure in a the wheel cylinder, in releasing mode for decreasing thefluid pressure in the wheel cylinder and in a holding mode for holdingthe fluid pressure in the wheel cylinder at a substantially constantlevel. One of the foregoing operational modes is selected depending uponthe braking condition derived based on a wheel speed, a wheelacceleration, a slip rate and so forth.

In U.S. Pat. No. 4,408,290, issued on Oct. 4, 1983 to the commoninventor and commonly assigned, discloses a method and system forsampling input time data for use in calculation of acceleration anddeceleration. In the disclosure of the applicant's prior invention, anacceleration sensor acts on the variable-frequency pulses of a speedsensor signal to recognize any variation of the pulse period thereof andto produce an output indicative of the magnitude of the detectedvariation to within a fixed degree of accuracy. The durations of groupsof pulses are held to within a fixed range by adjusting the number ofpulses in each group. The duration of groups of pulses are measured withreference to a fixed-frequency clock pulse signal and the measurementperiods of successive groups of equal numbers of pulses are compared. Ifthe difference between pulse group periods is zero or less than apredetermined value, the number of pulses in each group is increased inorder to increase the total number of clock pulses during themeasurement interval. The number of pulses per group is increased untilthe difference between measured periods exceeds the predetermined valueor until the number of pulses per group reaches a predetermined maximum.Acceleration data calculation and memory control procedure are designedto take into account the variation of the number of pulse per group.

The wheel speed is calculated periodically after the pulse interval orduration of the grouped sensor pulses becomes greater than apredetermined value. A slip rate is periodically derived from thederived wheel speed. On the other hand, the wheel acceleration iscalculated only when the difference between the sensor pulse intervalsor the durations of successive sensor pulse groups becomes larger than agiven value. Therefore, wheel acceleration may be derived later than thewheel speed and the slip rate. Since the operation mode is derivedperiodically corresponding to calculation timing of the wheel speed andslip rate, the delay in deriving the wheel acceleration value can causeerrors in selecting the operation mode.

In particularly significant cases, one of the operation modes in apredetermined skid control pattern may be skipped, i.e. not performedduring one cycle of the skid control operation. For example, it ispossible for the holding mode to be selected immediately after havingselected the holding mode, i.e. to skip either a release mode orapplication mode. In such case, if it is an application mode which isskipped, and thus the fluid pressure in the wheel cylinder is held at areduced value, wheel speed will increase non-optimally, resulting in anunnecessarily prolonged braking distance. On the other hand, if arelease mode period is skipped, the fluid pressure in the wheel cylinderwill be held at an excessively high value, increasing the slip ratenon-optimally, which may result in a skid.

Such danger is caused when one period of a scheduled operation modecycle is skipped.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide ananti-skid brake control system in which none of the scheduled operationmodes in a predetermined operation cycle can be skipped.

Another and more specific object of the invention is to provide ananti-skid brake control system, in which skipping of any of thescheduled operation modes in a predetermined operation pattern isdetected and operation is modified to perform the scheduled operationmode which otherwise would be skipped.

In order to achieve the above-mentioned and other objects, an anti-skidbrake control system according to the present invention controls theoperation mode of a pressure control valve in a hydraulic brake circuitfor increasing the fluid pressure in its application mode, fordecreasing the fluid pressure in its releasing mode and for holding thefluid pressure at a constant value in its holding mode. The controlsystem has a predetermined operation pattern in which the operationmodes of the pressure control valve cycle through a predeterminedschedule. Measured wheel acceleration and slip rate values dictate whichoperation modes are to be selected next and when the change from mode tomode should be performed. When the operation mode is to be changed, thenext operation mode selected is checked to see if it is consistent withthat scheduled. When the selected operation mode is different from thescheduled one, the selected operation mode is modified to perform theoperation according to the scheduled operation mode which wouldotherwise be skipped.

In accordance with one aspect of the invention, an anti-skid brakecontrol system for an automotive hydraulic brake system comprises ahydraulic brake circuit including a wheel cylinder for applying brakingforce to a vehicle wheel, a pressure control valve disposed within thehydraulic circuit and operative to increase fluid pressure in the wheelcylinder in its first position, to decrease the fluid pressure in thewheel cylinder in its second position and to hold the fluid pressure inthe wheel cylinder constant in its third position, a wheel speed sensormeans for detecting the rotational speed of the vehicle wheel andproducing sensor signal pulses separated by intervals representative ofthe detected wheel speed, a timer means for producing a timer signalhaving a value indicative of elapsed time, a first means for detectingwheel acceleration and producing a first signal indicative of thedetected wheel acceleration, a second means for detecting the slip rateof the vehicle wheel with respect to the ground and producing a secondsignal indicative of the detected slip rate, a third means for producinga control signal which actuates the pressure control valve to one of thefirst, second and third positions so as to adjust the wheel rotationalspeed toward an optimal relationship with vehicle speed, the third meansselecting the control signal according to predetermined relationshipsbetween the first and second signal values and according to apredetermined sequence of actuation of the first, second and thirdpositions, and a fourth means, associated with the third means, forchecking the control signal and when the control signal is not inaccordance with the predetermined sequence, modifying the control signalso as to accord with the predetermined sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of thepreferred embodiments of the present invention, which, however, shouldnot be taken to limit the invention to the specific embodiments, but arefor explanation and understanding only.

FIG. 1 is a schematic block diagram of the general design of thepreferred embodiment of an anti-skid brake control system according tothe present invention;

FIG. 2 is a perspective illustration of the hydraulic circuits of theanti-skid brake system according to the present invention;

FIG. 3 is a circuit diagram of the hydraulic circuits performing theanti-skid control according to the present invention;

FIG. 4 is an illustration of the operation of an electromagnetic flowcontrol valve employed in the hydraulic circuit, which valve has beenshown in an application mode for increasing the fluid pressure in awheel cylinder;

FIG. 5 is a view similar to FIG. 4 but of the valve in a hold mode inwhich the fluid pressure in the wheel cylinder is held at asubstantially constant value;

FIG. 6 is a view similar to FIG. 4 but of the valve in a release mode inwhich the fluid pressure in the wheel cylinder is reduced;

FIG. 7 is an elevation of a wheel speed sensor adapted to detect thespeed of a front wheel;

FIG. 8 is an elevation of a wheel speed sensor adapted to detect thespeed of a rear wheel;

FIG. 9 is an explanatory illustration of the wheel speed sensors ofFIGS. 7 and 8;

FIG. 10 is a timing chart for the anti-skid control system;

FIG. 11 is a block diagram of the preferred embodiment of a controllerunit in the anti-skid brake control system according to the presentinvention;

FIG. 12 is a flowchart of a main program of a microprocessorconstituting the controller unit of FIG. 11;

FIG. 13 is a flowchart of an interrupt program executed by thecontroller unit;

FIG. 14 is a flowchart of a main routine in the main program of FIG. 12;

FIG. 15 is a flowchart of an output calculation program for deriving EVand V signals for controlling operation mode of the electromagneticvalve according to the valve conditions of FIGS. 4, 5 and 6;

FIG. 16 is a table determining the operation mode of the actuator 16,which table is accessed in terms of the wheel acceleration and the sliprate;

FIG. 17 shows variation of derived wheel speed;

FIG. 18 shows variation of derived wheel acceleration when MODE 4 of thesample mode is selected;

FIG. 19 is a timing chart of calculation of the wheel speed and wheelacceleration performed by the anti-skid control system;

FIG. 20 shows a Lissajous figure illustrated in terms of the wheelacceleration and the slip rate in a normal skid control cycle;

FIG. 21 is a similar graph to FIG. 20 but showing deformation of theLissajous' figure due to delay of calculation of the wheel accelerationand deceleration;

FIG. 22 shows the variation of wheel speed and wheel acceleration asanti-skid brake control is performed, in which the solid line shows anormal state and the broken line shows a state in which one scheduledoperation mode is skipped;

FIG. 23 is a flowchart of a control or operation mode determiningroutine in the output calculation program of FIG. 15;

FIG. 24 is a block diagram of another embodiment of the controller unitin the preferred embodiment of the anti-skid brake control systemaccording to the present invention;

FIG. 25 is a block diagram of an output unit in the controller unit ofFIG. 24;

FIG. 26 is a block diagram of a modification of the output unit of FIG.25; and

FIG. 27 is a table determining the operation mode of the actuator 16,which table is accessed in terms of the wheel acceleration anddeceleration and the slip rate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This application is one of eighteen mutually related co-pending patentapplications in the United States, filed on the same day. All of theeighteen applications have been filed by the common applicant to thisapplication and commonly assigned to the assignee of this application.The other seventeen applications are identified below:

    __________________________________________________________________________    Basic Japanese                                                                Patent Appln No.                                                              (U.S. Serial No.)                                                                           Title of the Invention                                          __________________________________________________________________________    Showa 58-70891                                                                              AN AUTOMOTIVE ANTI-SKID BRAKE                                   (601,326, filed April 17, 1984)                                                             CONTROL SYSTEM WITH SAMPLING INPUT                                            TIME DATA OF WHEEL SPEED SENSOR                                               SIGNALS                                                         Showa 58-70892                                                                              METHOD AND SYSTEM FOR SAMPLING INPUT                            (601,375, filed April 17, 1984)                                                             TIME DATA FOR WHEEL SPEED SENSOR IN                                           AN AUTOMOTIVE ANTI-SKID BRAKE                                                 CONTROL SYSTEM                                                  Showa 58-70893                                                                              AUTOMOTIVE ANTI-SKID CONTROL SYSTEM                             (601,325, filed April 17, 1984)                                                             WITH CONTROL OF SAMPLING OF INPUT                                             TIME DATA OF WHEEL SPEED SENSOR                                               SIGNALS AND METHOD THEREFOR                                     Showa 58-70894                                                                              ANTI-SKID CONTROL SYSTEM FOR AUTO-                              (601,317, filed April 17, 1984)                                                             MOTIVE BRAKE SYSTEM WITH SAMPLE                                               CONTROL FOR SAMPLING INPUT TIMING OF                                          SENSOR SIGNAL PULSES WITH REQUIRED                                            PROCESS IDENTIFICATION AND METHOD                                             FOR SAMPLING                                                    Showa 58-70895                                                                              ANTI-SKID BRAKE CONTROL SYSTEM                                  (601,294, filed April 17, 1984)                                                             INCLUDING A PROCEDURE OF SAMPLING OF                                          INPUT TIME DATA OF WHEEL SPEED                                                SENSOR SIGNALS AND METHOD THEREFOR                              Showa 58-70896                                                                              ANTI-SKID BRAKE CONTROL SYSTEM                                  (601,344, filed April 17, 1984)                                                             INCLUDING WHEEL DECELERTION CALCU-                                            LATION WITH SHORTER LAG-TIME AND                                              METHOD FOR PERFORMING CALCULATION                               Showa 58-70897                                                                              ANTI-SKID BRAKE CONTROL SYSTEM WITH                             (601,338, filed April 17, 1984)                                                             SAMPLE CONTROL OF SENSOR SIGNAL                                               INPUT TIME DATA, AND METHOD THEREFOR                            Showa 58-70898                                                                              ANTI-SKID BRAKE CONTROL SYSTEM WITH                             (601,337, filed April 17, 1984)                                                             CONTROL OF SAMPLING TIMING OF INPUT                                           TIMING VALUES OF WHEEL SPEED SENSOR                                           SIGNAL PULSES                                                   Showa 58-70899                                                                              ANTI-SKID BRAKE CONTROL SYSTEM FOR                              (601,330, filed April 17, 1984)                                                             AUTOMOTIVE VEHICLE                                              Showa 58-70900                                                                              ANTI-SKID BRAKE CONTROL SYSTEM WITH                             (601,364, filed April 17, 1984)                                                             REDUCED DURATION OF WHEEL ACCELE-                                             RATION AND DECELERATION CALCULATION                             Showa 58-84087 &                                                                            ANTI-SKID BRAKE CONTROL SYSTEM WITH                             58-84091      OPERATION CONTROL FOR A PRESSURE                                (601,329, filed April 17, 1984)                                                             REDUCTION FLUID PUMP IN HYDRAULIC                                             BRAKE CIRCUIT                                                   Showa 58-84082                                                                              METHOD AND SYSTEM FOR DERIVING WHEEL                            (601,318, filed April 17, 1984)                                                             ROTATION SPEED DATA FOR AUTOMOTIVE                                            ANTI-SKID CONTROL                                               Showa 58-84085                                                                              METHOD AND SYSTEM FOR DERIVING WHEEL                            (601,345, filed April 17, 1984)                                                             ACCELERATION AND DECELERATION IN                                              AUTOMOTIVE ANTI-SKID BRAKE CONTROL                                            SYSTEM                                                          Showa 58-84092                                                                              ANTI-SKID BRAKE CONTROL SYSTEM AND                              (601,293, filed April 17, 1984)                                                             METHOD FEATURING VEHICLE BATTERY                                              PROTECTION                                                      Showa 58-84081                                                                              METHOD AND SYSTEM FOR DERIVING WHEEL                            (601,327, filed April 17, 1984)                                                             ROTATION SPEED DATA FOR AUTOMOTIVE                                            ANTI-SKID CONTROL                                               Showa 58-84090                                                                              ANTI-SKID BRAKE CONTROL SYSTEM                                  (601,258, filed April 17, 1984,                                                             INCLUDING FLUID PUMP AND DRIVE                                  now Patent No. 4,569,560                                                                    CIRCUIT THEREFOR                                                issued February 11, 1986)                                                     Shown 58-102919 &                                                                           ANTI-SKID BRAKE CONTROL SYSTEM WITH                             58-109308     A PLURALITY OF INDEPENDENTLY                                    (601,295, filed April 17, 1984)                                                             OPERATIVE DIGITAL CONTROLLERS                                   __________________________________________________________________________

Disclosures of other seventeen applications as identified above arehereby incorporated by reference for the sake of disclosure.

Referring now to the drawings, particularly to FIG. 1, the preferredembodiment of an anti-skid control system according to the presentinvention includes a control module 200 including a front-leftcontroller unit (FL) 202, a front-right controller unit (FR) 204 and arear controller unit (R) 206. The controller unit 202 comprises amicroprocessor and is adapted to control brake pressure applied to afront left wheel cylinder 30a of a front left hydraulic brake system 302of an automotive hydraulic brake system 300. Similarly, the controllerunit 204 is adapted to control brake pressure applied to the wheelcylinder 34a of a front right wheel (not shown) in the front righthydraulic brake system 304 and the controller unit 206 is adapted tocontrol brake pressure applied to the rear wheel cylinders 38a of thehydraulic rear brake system 306. Respective brake systems 302, 304 and306 have electromagnetically operated actuators 16, 18 and 20, each ofwhich controls the pressure of working fluid in the corresponding wheelcylinders. By means of the controlled pressure, the wheel cylinders 30a,34a and 38a apply braking force to brake disc rotors 28, 32 and 36mounted on the corresponding wheel axles for rotation with thecorresponding vehicle wheels via brake shoe assemblies 30, 34 and 38.

Though the shown brake system comprises disc brakes, the anti-skidcontrol system according to the present invention can also be applied todrum-type brake systems.

The controller units 202, 204 and 206 are respectively associated withactuator drive circuits 214, 216 and 218 to control operations ofcorresponding actuators 16, 18 and 20. In addition, each of thecontroller units 202, 204 and 206 is connected to a corresponding wheelspeed sensor 10, 12 and 14 via shaping circuits 208, 210 and 212incorporated in the controller 200. Each of the wheel speed sensors 10,12 and 14 is adapted to produce an alternating-current sensor signalhaving a frequency related to or proportional to the rotation speed ofthe corresponding vehicle wheel. Each of the A-C sensor signals isconverted by the corresponding shaping circuit 208, 210 and 212 into arectangular pulse signal which will be hereafter referred to as "sensorpulse signal". As can be appreciated, since the frequency of the A-Csensor signals is proportional to the wheel speed, the frequency of thesensor pulse signal should correspond to the wheel rotation speed andthe pulse intervals thereof will be inversely proportional to the wheelrotation speed.

The controller units 202, 204 and 206 operate independently andcontinuously process the sensor pulse signal to derive control signalsfor controlling the fluid pressure in each of the wheel cylinders 30a,34a and 38a in such a way that the slip rate R at each of the vehiclewheels is optimized to shorten the distance required to stop thevehicle, which distance will be hereafter referred to as "brakingdistance".

In general, each controller unit 202, 204 and 206 monitors receipt ofthe corresponding sensor pulses so that it can derive the pulse intervalbetween the times of receipt of successive sensor pulses. Based on thederived pulse interval, the controller units 202, 204 and 206 calculateinstantaneous wheel speed V_(w) and instantaneous wheel acceleration ordeceleration a_(w). From these measured and derived values, a targetwheel speed V_(i) is derived, which is an assumed value derived from thewheel speed at which a slip is assumed to be zero or approximately zero.The target wheel speed V_(i) varies at a constant decelerating ratederived from variation of the wheel speed. The target wheel speed thuscorresponds to a vehicle speed which itself is based on variations ofthe wheel speed. Based on the difference between the instantaneous wheelspeed V_(w) and the target wheel speed V_(i), a slip rate R is derived.The controller units 202, 204 and 206 determine the appropriateoperational mode for increasing, decreasing or holding the hydraulicbrake pressure applied to the wheel cylinders 30a, 34a and 38 a. Thecontrol mode in which the brake pressure is increased will be hereafterreferred to as "application mode". The control mode in which the brakepressure is decreased will be hereafter referred to as "release mode".The mode in which the brake pressure is held essentially constant willbe hereafter referred to as "hold mode". The anti-skid control operationconsists of a loop of the application mode, hold mode, release mode andhold mode. This loop is repeated throughout the anti-skid brake controloperation cyclically. One cycle of the loop of the control variationwill be hereafter referred to as "skid cycle".

FIG. 2 shows portions of the hydraulic brake system of an automotivevehicle to which the preferred embodiment of the anti-skid controlsystem is applied. The wheel speed sensors 10 and 12 are respectivelyprovided adjacent the brake disc rotor 28 and 32 for rotation therewithso as to produce sensor signals having frequencies proportional to thewheel rotation speed and variable in accordance with variation of thewheel speed. On the other hand, the wheel speed sensor 14 is providedadjacent a propeller shaft near the differential gear box or drivepinion shaft 116 for rotation therewith. Since the rotation speeds ofthe left and right rear wheels are free to vary independently dependingupon driving conditions due to the effect of the differential gear box40, the rear wheel speed detected by the rear wheel speed sensor 14 isthe average of the speeds of the left and right wheels. Throughout thespecification, "rear wheel speed" will mean the average rotation speedof the left and right rear wheels.

As shown in FIG. 2, the actuator unit 300 is connected to a master wheelcylinder 24 via primary and secondary outlet ports 41 and 43 thereof andvia pressure lines 44 and 42. The master wheel cylinder 24 is, in turn,associated with a brake pedal 22 via a power booster 26 which is adaptedto boost the braking force applied to the brake pedal 22 before applyingsame to the master cylinder. The actuator unit 300 is also connected towheel cylinders 30a, 34a and 38a via brake pressure lines 46, 48 and 50.

The circuit lay-out of the hydraulic brake system circuit will bedescribed in detail below with reference to FIG. 3 which is only anexample of the hydraulic brake system to which the preferred embodimentof the anti-skid control system according to the present invention canbe applied, and so it should be appreciated that it is not intended tolimit the hydraulic system to the embodiment shown. In FIG. 3, thesecondary outlet port 43 is connected to the inlet ports 16b and 18b ofelectromagnetic flow control valves 16a and 18a, the respective outletports 16c and 18c of which are connected to corresponding left and rightwheel cylinders 30a and 34a, via the secondary pressure lines 46 and 48.The primary outlet port 41 is connected to the inlet port 20b of theelectromagnetic valve 20a, the outlet port 20c of which is connected tothe rear wheel cylinders 38a, via the primary pressure line 50. Theelectromagnetic valves 16a, 18a and 20a also have drain ports 16d, 18dand 20d. The drain ports 16d and 18d are connected to the inlet port 72aof a fluid pump 90 via drain passages 80, 82 and 78. The fluid pump 90is associated with an electric motor 88 to be driven by the latter whichis, in turn, connected to a motor relay 92, the duty cycle of which iscontrolled by means of a control signal from the control module 200.While the motor relay 92 is energized to be turned ON, the motor 88 isin operation to drive the fluid pump 90. The drain port 20d of theelectromagnetic flow control valve 20a is connected to the inlet port58a of the fluid pump 90 via a drain passage 64.

The outlet ports 72b and 58b are respectively connected to the pressurelines 42 and 44 via a return passages 72c and 58c. The outlet ports 16c,18c and 20c of respective electromagnetic flow control valves 16a, 18aand 20a are connected to corresponding wheel cylinders 30a, 34a and 38avia braking lines 46, 48 and 50. Bypass passages 96 and 98 are providedto connect the braking pressure lines 46 and 48, and 50 respectively tothe pressure lines 42 and 44, bypassing the electromagnetic flow controlvalves.

Pump pressure check valves 52 and 66 are installed in the pressure lines42 and 44. Each of the pump pressure check valves 66 and 52 is adaptedto prevent the working fluid pressurized by the fluid pump 90 fromtransmitting pressure surges to the master cylinder 24. Since the fluidpump 90 is designed for quick release of the braking pressure in thebraking pressure lines 46, 48 and 50 and thus releasing the wheelcylinders 30a, 34a and 38a from the braking pressure, it is driven uponrelease of the brake pedal. This would result in pressure surges in theworking fluid from the fluid pump 90 to the master cylinder 24 if thepump pressure check valves 66 and 52 were not provided. The pumppressure check valves 66 and 52 serve as one-way check valves allowingfluid flow from the master cylinder 24 to the inlet ports 16b, 18b and20b of the electromagnetic valves 16a, 18a and 20a. Pressureaccumulators 70 and 56 are installed in the pressure lines 42 and 44,which pressure accumulators serve to accumulate fluid pressure generatedat the outlet ports 72b and 58b of the fluid pump 90 while the inletports 16b, 18b and 20b are closed. Toward this end, the pressureaccumulators 70 and 56 are connected to the outlet ports 72b and 58b ofthe fluid pump 90 via the return passages 72c and 58c. Outlet valves 68and 54 are one-way check valves allowing one-way fluid communicationfrom the fluid pump to the pressure accumulators. These outlet valves 68and 54 are effective for preventing the pressure accumulated in thepressure accumulators 70 and 56 from surging to the fluid pump when thepump is deactivated. In addition, the outlet valves 68 and 54 are alsoeffective to prevent the pressurized fluid flowing through the pressurelines 42 and 44 from flowing into the fluid pump 90 through the returnpassages 72c and 58c.

Inlet check valves 74 and 60 are inserted in the drain passages 78 and64 for preventing surge flow of the pressurized fluid in the fluid pump90 to the electromagnetic flow control valves 16a, 18a and 20a after thebraking pressure in the wheel cylinders is released. The fluid flowingthrough the drain passages 78 and 64 is temporarily retained in fluidreservoirs 76 and 62 connected to the former.

Bypass check valves 85, 86 and 84 are inserted in the bypass passages 98and 96 for preventing the fluid in the pressure lines 42 and 44 fromflowing to the braking pressure lines 46, 48 and 50 without firstpassing through the electromagnetic flow control valves 16a, 18a and20a. On the other hand, the bypass check valves 85, 86 and 84 areadapted to permit fluid flow from the braking pressure line 46, 48 and50 to the pressure lines 42 and 44 when the master cylinder 24 isreleased and thus the line pressure in the pressure lines 42 and 44becomes lower than the pressure in the braking pressure lines 46, 48 and50.

The electromagnetic flow control valves 16a, 18a and 20a arerespectively associated with the actuators 16, 18 and 20 to becontrolled by means of the control signals from the control module 200.The actuators 16, 18 and 20 are all connected to the control module 200via an actuator relay 94, which thus controls the energization anddeenergization of them all. Operation of the electromagnetic valve 16ain cooperation with the actuator 16 will be illustrated with referenceto FIGS. 4, 5 and 6, in particular illustrating the application mode,hold mode and release mode, respectively.

It should be appreciated that the operation of the electromagneticvalves 18a and 20a are substantially the same as that of the valve 16a.Therefore, disclosure of the valve operations of the electromagneticvalves 18a and 20a is omitted in order to avoid unnecessary repetitionand for simplification of the disclosure.

APPLICATION MODE

In this position, the actuator 16 remains deenergized. An anchor of theelectromagnetic valve 16a thus remains in its initial position allowingfluid flow between the inlet port 16b and the outlet port 16c so thatthe pressurized fluid supplied from the master cylinder 24 via thepressure line 42 may flow to the left front wheel cylinder 30a via thebraking pressure line 46. In this valve position, the drain port 16d isclosed to block fluid flow from the pressure line 42 to the drainpassage 78. As a result, the line pressure in the braking pressure line46 is increased in proportion to the magnitude of depression of thebrake pedal 22 and thereby the fluid pressure in the left front wheelcylinder 30a is increased correspondingly.

In this case, when the braking force applied to the brake pedal isreleased, the line pressure in the pressure line 42 drops due to returnof the master cylinder 24 to its initial position. As a result, the linepressure in the braking pressure line 46 becomes higher than that in thepressure line 42 and so opens the bypass valve 85 to permit fluid flowthrough the bypass passage 98 to return the working fluid to the fluidreservoir 24a of the master cylinder 24.

In the preferring construction, the pump pressure check valve 66,normally serving as a one-way check valve for preventing fluid flow fromthe electromagnetic valve 16a to the master cylinder 24, becomeswide-open in response to drop of the line pressure in the pressure linebelow a given pressure. This allows the fluid in the braking pressureline 46 to flow backwards through the electromagnetic valve 16a and thepump pressure check valve 66 to the master cylinder 24 via the pressureline 42. This function of the pump pressure check valve 66 facilitatesfull release of the braking pressure in the wheel cylinder 30a.

For instance, the bypass valve 85 is rated at a given set pressure, e.g.2 kg/cm² and closes when the pressure difference between the pressureline 42 and the braking pressure line 46 drops below the set pressure.As a result, fluid pressure approximating the bypass valve set pressuretends to remain in the braking pressure line 46, preventing the wheelcylinder 30a from returning to the fully released position. In order toavoid this, in the shown embodiment, the one-way check valve function ofthe pump pressure check valve 66 is disabled when the line pressure inthe pressure line 42 drops below a predetermined pressure, e.g. 10kg/cm². When the line pressure in the pressure line 42 drops below thepredetermined pressure, a bias force normally applied to the pumppressure check valve 66 is released, freeing the valve to allow fluidflow from the braking pressure line 46 to the master cylinder 24 via thepressure line 42.

HOLD MODE

In this control mode, a limited first value, e.g. 2A of electric currentserving as the control signal is applied to the actuator 16 to positionthe anchor closer to the actuator 16 than in the previous case. As aresult, the inlet port 16b and the drain port 16d are closed to blockfluid communication between the pressure line 42 and the brakingpressure line 46 and between the braking pressure line and the drainpassage 78. Therefore, the fluid pressure in the braking pressure line46 is held at the level extant at the moment the actuator is operated bythe control signal.

In this case, the fluid pressure applied through the master cylinderflows through the pressure check valve 66 to the pressure accumulator70.

RELEASE MODE

In this control mode, a maximum value, e.g. 5A of electric currentserving as the control signal is applied to the actuator 16 to shift theanchor all the way toward the actuator 16. As a result, the drain port16d is opened to establish fluid communication between the drain port16d and the outlet port 16c. At this time, the fluid pump 90 serves tofacilitate fluid flow from the braking pressure line 46 to the drainpassage 78. The fluid flowing through the drain passage is partlyaccumulated in the fluid reservoir 76 and the remainder flows to thepressure accumulator 70 via the check valves 60 and 54 and the fluidpump 90.

It will be appreciated that, even in this release mode, the fluidpressure in the pressure line 42 remains at a level higher or equal tothat in the braking pressure line 46, so that fluid flow from thebraking pressure line 46 to the pressure line 42 via the bypass passage98 and via the bypass check valve 85 will never occur.

In order to resume the braking pressure in the wheel cylinder (FL) 30aafter once the braking pressure is reduced by positioning theelectromagnetic valve 16a in the release position, the actuator 16 isagain deenergized. The electromagnetic valve 16a is thus returns to itsinitial position to allow the fluid flow between the inlet port 16b andthe outlet port 16c so that the pressurized fluid may flow to the leftfront wheel cylinder 30a via the braking pressure line 46. As set forththe drain port 16d is closed to block fluid flow from the pressure line42 to the drain passage 78.

As a result, the pressure accumulator 70 is connected to the left frontwheel cylinder 30a via the electromagnetic valve 16a and the brakingpressure line 46. The pressurized fluid in the pressure accumulator 70is thus supplied to the wheel cylinder 30a so as to resume the fluidpressure in the wheel cylinder 30a.

At this time, as the pressure accumulator 70 is connected to the fluidreservoir 76 via the check valves 60 and 54 which allow fluid flow fromthe fluid reservoir to the pressure accumulator, the extra amount ofpressurized fluid may be supplied from the fluid reservoir.

The design of the wheel speed sensors 10, 12 and 14 employed in thepreferred embodiment of the anti-skid control system will be describedin detail with reference to FIGS. 7 to 9.

FIG. 7 shows the structure of the wheel speed sensor 10 for detectingthe rate of rotation of the left front wheel. The wheel speed sensor 10generally comprises a sensor rotor 104 adapted to rotate with thevehicle wheel, and a sensor assembly 102 fixedly secured to the shimportion 106 of the knuckle spindle 108. The sensor rotor 104 is fixedlysecured to a wheel hub 109 for rotation with the vehicle wheel.

As shown in FIG. 9, the sensor rotor 104 is formed with a plurality ofsensor teeth 120 at regular angular intervals. The width of the teeth120 and the grooves 122 therebetween are equal in the shown embodimentand define a unit angle of wheel rotation. The sensor assembly 102comprises a magnetic core 124 aligned with its north pole (N) near thesensor rotor 104 and its south pole (S) distal from the sensor rotor. Ametal element 125 with a smaller diameter section 125a is attached tothe end of the magnetic core 124 nearer the sensor rotor. The free endof the metal element 125 faces the sensor teeth 120. An electromagneticcoil 126 encircles the smaller diameter section 125a of the metalelement. The electromagnetic coil 126 is adapted to detect variations inthe magnetic field generated by the magnetic core 124 to produce analternating-current sensor signal. That is, the metal element and themagnetic core 124 form a kind of proximity switch which adjusts themagnitude of the magnetic field depending upon the distance between thefree end of the metal element 125 and the sensor rotor surface. Thus,the intensity of the magnetic field fluctuates in relation to thepassage of the sensor teeth 120 and accordingly in relation to theangular velocity of the wheel.

It should be appreciated that the wheel speed sensor 12 for the rightfront wheel has the substantially the same structure as the set forthabove. Therefore, explanation of the structure of the right wheel speedsensor 12 will be omitted in order to avoid unnecessary repetition inthe disclosure and in order to simplify the description.

FIG. 8 shows the structure of the rear wheel speed sensor 14. As withthe aforementioned left front wheel speed sensor 10, the rear wheelspeed sensor 14 comprises a sensor rotor 112 and a sensor assembly 102.The sensor rotor 112 is associated with a companion flange 114 which is,in turn, rigidly secured to a drive shaft 116 for rotation therewith.Thus, the sensor rotor 112 rotates with the drive shaft 116. The sensorassembly 102 is fixed to a final drive housing or a differential gearbox (not shown).

Each of the sensor assemblies applied to the left and right front wheelspeed sensors and the rear wheel sensor is adapted to output analternating-current sensor signal having a frequency proportional to orcorresponding to the rotational speed of the corresponding vehiclewheel. The electromagnetic coil 126 of each of the sensor assemblies 102is connected to the control module 200 to supply the sensor signalsthereto.

As set forth above, the control module 200 comprises the controller unit(FL) 202, the controller unit (FR) 204 and the controller unit (R) 206,each of which comprises a microcomputer. Therefore, the wheel speedsensors 10, 12 and 14 are connected to corresponding controller units202, 204 and 206 and send their sensor signals thereto. Since thestructure and operation of each of the controller units is substantiallythe same as that of the others, the structure and operation of only thecontroller unit 202 for performing the anti-skid brake control for thefront left wheel cylinder will be explained in detail.

FIG. 10 is a timing chart of the anti-skid control performed by thecontroller unit 202. As set forth above, the alternating-current sensorsignal output from the wheel speed sensor 10 is converted into arectangular pulse train, i.e. as the sensor pulse signal mentionedabove. The controller unit 202 monitors occurrences of sensor pulses andmeasures the intervals between individual pulse or between the firstpulses of groups of relatively-high-frequency pulses. Pulses are sogrouped that the measured intervals will exceed a predetermined value,which value will be hereafter referred to as "pulse interval threshold".

The wheel rotation speed V_(w) is calculated in response to each sensorpulse. As is well known, the wheel speed is generally inverselyproportional to the intervals between the sensor pulses, andaccordingly, the wheel speed V_(w) is derived from the interval betweenthe last sensor pulse input time and the current sensor pulse inputtime. A target wheel is designated V_(i) and the resultant wheel speedis designated V_(w). In addition, the slip rate is derived from the rateof change of the wheel speed and an projected speed V_(v) which isestimated from the wheel speed at the moment the brakes are appliedbased on the assumption of a continuous, linear deceleration withoutslippage. In general, the target wheel speed V_(i) is derived from thewheel speed of the last skid cycle during which the wheel decelerationrate was equal to or less than a given value which will be hereafterreferred to as "deceleration threshold a_(ref) ", and the wheel speed ofthe current skid cycle, and by estimating the rate of change of thewheel speed between wheel speeds at which the rate of deceleration isequal to or less than the deceleration threshold. In practice, the firsttarget wheel speed V_(i) is derived based on the projected speed V_(v)which corresponds to a wheel speed at the initial stage of brakingoperation and at which wheel deceleration exceeds a predetermined value,e.g. -1.2G, and a predetermined deceleration rate, for example 0.4G. Thesubsequent target wheel speed V_(i) is derived based on the projectedspeeds V_(v) in last two skid cycles. For instance, the decelerationrate of the target wheel speed V_(i) is derived from a difference of theprojected speeds V_(v) in the last two skid cycle and a period of timein which wheel speed varies from the first projected speed to the nextprojected speed. Based on the last projected speed and the decelerationrate, the target wheel speed in the current skid cycle is derived.

The acceleration and deceleration of the wheel is derived based on theinput time of three successive three sensor pulses. Since the intervalof the adjacent sensor signal pulses corresponds to the wheel speed, andthe wheel speed is a function of the reciprocal of the interval, bycomparing adjacent pulse-to-pulse intervals, a value corresponding tothe variation or difference of the wheel speed may be obtained. Theresultant interval may be divided by the period of time of the intervalin order to obtain the wheel acceleration and deceleration at the unittime. Therefore, the acceleration or deceleration of the wheel isderived from the following equation: ##EQU1## where A, B and C are theinput times of the sensor pulses in the order given.

On the other hand, the slip rate R is a rate of difference of wheelspeed relative to the vehicle speed which is assumed as substantiallycorresponding to the target wheel speed. Therefore, in the shownembodiment, the target wheel speed V_(i) is taken as variable orparameter indicative of the assumed or projected vehicle speed. The sliprate R can be obtained by dividing a difference between the target wheelspeed V_(i) and the instantaneous wheel speed V_(w) by the target wheelspeed. Therefore, in addition, the slip rate R is derived by solving thefollowing equation: ##EQU2##

Finally, the controller unit 202 determines the control mode, i.e.,release mode, hold mode and application mode from the slip rate R andthe wheel acceleration or deceleration a_(w).

General operation of the controller unit 202 will be briefly explainedherebelow with reference to FIG. 11. Assuming the brake is applied at t₀and the wheel deceleration a_(w) exceeds the predetermined value, e.g.1.2G at a time t₁, the controller unit 202 starts to operate at a timet₁. The first sensor pulse input time (t₁) is held in the controllerunit 202. Upon receipt of the subsequent sensor pulse at a time t₂, thewheel speed V_(w) is calculated by deriving the current sensor pulseperiod (dt=t₂ -t₁). In response to the subsequently received sensorpulses at times t₃, t₄ . . . , the wheel speed values V_(w2), V_(w3) . .. are calculated.

On the other hand, at the time t₁, the instantaneous wheel speed istaken as the projected speed V_(v). Based on the projected speed V_(v)and the predetermined fixed value, e.g. 0.4G, the target wheel speedV_(i) decelerating at the predetermined deceleration rate 0.4G isderived.

In anti-skid brake control, the braking force applied to the wheelcylinder is to be so adjusted that the peripheral speed of the wheel,i.e. the wheel speed, during braking is held to a given ratio. e.g. 85%to 80% of the vehicle speed. Therefore, the slip rate R has to bemaintained below a given ratio, i.e., 15% to 10%. In the preferredembodiment, the control system controls the braking force so as tomaintain the slip rate at about 15%. Therefore, a reference valueR_(ref) to be compared with the slip rate R is determined at a value at85% of the projected speed V_(v). As will be appreciated, the referencevalue is thus indicative of a slip rate threshold, which will behereafter referred to "slip rate threshold R_(ref) " throughout thespecification and claims, and varies according to variation of thetarget wheel speed.

In practical brake control operation performed by the preferredembodiment of the anti-skid control system according to the presentinvention, the electric current applied to the actuator attains alimited value, e.g., 2A to place the electromagnetic valve 30a in thehold mode as shown in FIG. 5 when the wheel speed remains inbetween thetarget wheel speed V_(i) and the slip rate threshold R_(ref). When theslip rate derived from the target wheel speed V_(i) and the wheel speedV_(w) becomes equal to or larger than the slip rate threshold R_(ref),then the supply current to the actuator 16 is increased to a maximumvalue, e.g. 5A to place the electromagnetic valve in the release mode asshown in FIG. 6. By maintaining the release mode, the wheel speed V_(w)is recovered to the target wheel speed. When the wheel speed is thusrecovered or resumed so that the slip rate R at that wheel speed becomesequal to or less than the slip rate threshold R_(ref), then the supplycurrent to the actuator 16 is dropped to the limited value, e.g. 2A toreturn the electromagnetic valve 30a to the hold mode. By holding thereduced fluid pressure in the wheel cylinder, the wheel speed V_(w) isfurther resumed to the target wheel speed V_(i). When the wheel speedV_(w) is resumed equal to or higher than the target wheel speed V_(i),the supply current is further dropped to zero for placing theelectromagnetic valve in the application mode as shown in FIG. 4. Theelectromagnetic valve 30a is maintained in the application mode untilthe wheel speed is decelerated at a wheel speed at which the wheeldeceleration becomes equal to or slightly more than the decelerationthreshold R_(ref) -1.2G. At the same time, the projected speed V_(v) isagain derived with respect to the wheel speed at which the wheeldeceleration a_(w) becomes equal to or slightly larger than thedeceleration threshold a_(ref). From a difference of speed of the lastprojected speed and the instant projected speed and the period of timefrom a time obtaining the last projected speed to a time obtaining theinstant projected speed, a decleration rate of the target wheel speedV_(i) is derived. Therefore, assuming the last projected speed isV_(v1), the instant projected speed is V_(v2), and the period of time isT_(v), the target wheel speed V_(i) can be obtained from the followingequation:

    V.sub.i =V.sub.v2 -(V.sub.v1 -V.sub.v2)/T.sub.v ×t.sub.e

where t_(e) is an elapsed time from the time at which the instantprojected speed V_(v2) is obtained.

Based on the input timing to t₁, t₂, t₃, t₄ . . . , deceleration ratea_(w) is derived from the foregoing equation (1). In addition, theprojected speed V_(w) is estimated as a function of the wheel speedV_(w) and rate of change thereof. Based on the instantaneous wheelspeeds V_(w1) at which the wheel deceleration is equal to or less thanthe deceleration threshold a_(ref) and the predetermined fixed value,e.g. 0.4G for the first skid cycle of control operation, the targetwheel speed V_(i) is calculated. According to equation (2), the sliprate R is calculated, using successive wheel speed values V_(w1),V_(w2), V_(w3) . . . as parameters. The derived slip rate R is comparedwith the slip rate threshold R_(ref). As the wheel speed V_(w) dropsbelow the projected speed V_(v) at the time t₁, the controller unit 202switches the control mode from the application mode to the hold mode.Assuming also that the slip rate R exceeds the slip rate threshold atthe time t₄, then the controller unit 202 switches the control mode tothe release mode to release the fluid pressure at the wheel cylinder.

Upon release of the brake pressure in the wheel cylinder, the wheelspeed V_(w) recovers, i.e. the slip rate R drops until it is smallerthan the slip rate threshold at time t₇. The controller unit 202 detectswhen the slip rate R is smaller than the slip rate threshold R_(ref) andswitches the control mode from release mode to the hold mode.

By maintaining the brake system in the hold mode in which reduced brakepressure is applied to the wheel cylinder, the wheel speed increasesuntil it reaches the projected speed as indicated by the intersection ofthe dashed line (V_(v)) and the solid trace in the graph of V_(w) inFIG. 11. When the wheel speed V_(w) becomes equal to the target wheelspeed V_(i) (at a time t₈), the controller unit 202 switches the controlmode from the hold mode to the application mode.

As can be appreciated from the foregoing description, the control modewill tend to cycle through the control modes in the order applicationmode, hold mode, release mode and hold mode, as exemplified in theperiod of time from t₁ to t₈. This cycle of variation of the controlmodes will be referred to hereafter as "skid cycle". Practicallyspeaking, there will of course be some hunting and other minordeviations from the standard skid cycle.

The projected speed V_(v), which is meant to represent ideal vehiclespeed behavior, at time t₁ can be obtained directly from the wheel speedV_(w) at that time since zero slip is assumed. At the same time, thedeceleration rate of the vehicle will be assumed to be a predeterminedfixed value or the appropriate one of a family thereof, in order toenable calculation of the target wheel speed for the first skid cycleoperation. Specifically, in the shown example, the projected speed V_(v)at the time t₁ will be derived from the wheel speed V_(w1) at that time.Using the predetermined deceleration rate, the projected speed will becalculated at each time the wheel deceleration a_(w) in the applicationmode reaches the deceleration threshold a_(ref).

At time t₉, the wheel deceleration a_(w) becomes equal to or slightlylarger than the deceleration threshold a_(ref), then the secondprojected speed V_(v2) is obtained at a value equal to the instantaneouswheel speed V_(w) at the time t₉. According to the above-mentionedequation, the deceleration rate da can be obtained

    da=(V.sub.v1 -V.sub.v2)/(t.sub.9 -t.sub.1)

Based on the derived deceleration rate da, the target wheel speed V_(i)' for the second skid cycle of control operation is derived by:

    V.sub.i '=V.sub.v2 -da×t.sub.e

Based on the derived target wheel speed, the slip rate threshold R_(ref)for the second cycle of control operation is also derived. As will beappreciated from FIG. 11, the control mode will be varied during thesecond cycle of skid control operation, to hold mode at time t₉ at whichthe wheel deceleration reaches the deceleration threshold a_(ref) as setforth above, to release mode at time t₁₀ at which the slip rate Rreaches the slip rate threshold R_(ref), to hold mode at time t₁₁ atwhich the slip rate R is recovered to the slip rate threshold R_(ref),and to application mode at time t₁₂ at which the wheel speed V_(w)recovered or resumed to the target wheel speed V_(i) '. Further, itshould be appreciated that in the subsequent cycles of the skid controloperations, the control of the operational mode of the electromagneticvalve as set forth with respect to the second cycle of controloperation, will be repeated.

Relating the above control operations to the structure of FIGS. 3through 6, when application mode is used, no electrical current isapplied to the actuator of the electromagnetic valve 16a so that theinlet port 16b communicates with the outlet port 16c, allowing fluidflow between the pressure passage 42 and the brake pressure line 46. Alimited amount of electrical current (e.g. 2A) is applied at times t₁,t₇, t₉ and t₁₁, so as to actuate the electromagnetic valve 16a to itslimited stroke position by means of the actuator 16, and the maximumcurrent is applied to the actuator as long as the wheel speed V_(w) isnot less than the projected speed and the slip rate is greater than theslip rate threshold R_(ref). Therefore, in the shown example, thecontrol mode is switched from the application mode to the hold mode attime t₁ and then to the release mode at time t₄. At time t₇, the sliprate increases back up to the slip rate threshold R_(ref), so that thecontrol mode returns to the hold mode, the actuator driving theelectromagnetic valve 16a to its central holding position with thelimited amount of electrical current as the control signal. When thewheel speed V_(w) finally returns to the level of the target wheel speedV_(i) at time t₈, the actuator 16 supply current is cut off so that theelectromagnetic valve 16a returns to its rest position in order toestablish fluid communication between the pressure line 42 and thebraking pressure line 46 via inlet and outlet ports 16b and 16c.

Referring to FIG. 11, the controller unit 202 includes an inputinterface 230, CPU 232, an output interface 234, RAM 236 and ROM 238.The input interface 230 includes an interrupt command generator 229which produces an interrupt command in response to every sensor pulse.In ROM, a plurality of programs including a main program (FIG. 12), aninterrupt program (FIG. 14), an sample control program, a timer overflowprogram and an output calculation program (FIG. 15) are stored inrespectively corresponding address blocks 244, 246, 250, 252 and 254.

The input interface also has a temporary register for temporarilyholding input timing for the sensor pulses. RAM 236 similarly has amemory block holding input timing for the sensor pulses. The contents ofthe memory block 240 of RAM may be shifted whenever calculations of thepulse interval, wheel speed, wheel acceleration or deceleration, targetwheel speed, slip rate and so forth are completed. One method ofshifting the contents is known from the corresponding disclosure of theU.S. Pat. No. 4,408,290. The disclosure of the U.S. Pat. No. 4,408,290is hereby incorporated by reference. RAM also has a memory block 242 forholding pulse intervals of the input sensor pulses. The memory block 242is also adapted to shift the contents thereof according to the mannersimilar to set forth in the U.S. Pat. No. 4,408,290.

An interrupt flag 256 is provided in the controller unit 202 forsignalling interrupt requests to the CPU. The interrupt flag 256 is setin response to the interrupt command from the interrupt commandgenerator 229. A timer overflow interrupt flag 258 is adapted to set anoverflow flag when the measured interval between any pair of monitoredsensor pulses exceeds the capacity of a clock counter.

In order to time the arrival of the sensor pulses, a clock is connectedto the controller unit 202 to feed time signals indicative of elapsedreal time. The timer signal value is latched whenever a sensor pulse isreceived and stored in either or both of the temporary register 231 inthe input interface 230 and the memory block 240 of RAM 236.

The operation of the controller unit 202 and the function of eachelements mentioned above will be described with reference to FIGS. 12 to27.

FIG. 12 illustrates the main program for the anti-skid control system.Practically speaking, this program will generally be executed as abackground job, i.e. it will have a lower priority than most otherprograms under the control of the same processor. Its first step 1002 isto wait until at least one sample period, covering a single sensor pulseor a group thereof, as described in more detail below, is completed asindicated when a sample flag FL has a non-zero value. In subsequent step1004, the sample flag FL is checked for a value greater than one, whichwould indicate the sample period is too short. If this is the case,control passes to a sample control program labelled "1006" in FIG. 12.If FL=1, then the control process is according to plan, and controlpasses to a main routine explained later with reference to FIG. 14.Finally, after completion of the main routine, a time overflow flag OFLis reset to signify successful completion of another sample processingcycle, and the main program ends.

FIG. 13 shows the interrupt program stored in the memory block 246 ofROM 238 and executed in response to the interrupt command generated bythe interrupt command generator 229 whenever a sensor pulse is received.It should be noted that a counter value NC of an auxiliary counter 233is initially set to 1, a register N representing the frequency dividerratio is set at 1, and a counter value M of an auxiliary counter 235 isset at -1. After starting execution of the interrupt program, thecounter value NC of the auxiliary counter 233 is decremented by 1 at ablock 3002. The auxiliary counter value NC is then checked at a block3004 for a value greater than zero. For the first sensor pulse, sincethe counter value NC is decremented by 1 (1-1=0) at the block 3002 andthus is zero, the answer of the block 3004 is NO. In this case, theclock counter value t is latched in a temporary register 231 in theinput interface 230 at a block 3006. The counter value NC of theauxiliary counter 233 is thereafter assigned the value N in a register235, which register value N is representative of frequency dividingratio determined during execution of the main routine explained later,at a block 3008. The value M of an auxiliary counter 235 is thenincremented by 1. The counter value M of the auxiliary counter 235labels each of a sequence of sample periods covering an increasingnumber of sensor pulses. After this, the sample flag FL is incrementedby 1 at a block 3012. After the block 3012, interrupt program ends,returning control to the main program or back to block 3002, whichevercomes first.

On the other hand, when the counter value NC is non-zero when checked atthe block 3004, this indicates that not all of the pulses for thissample period have been received, and so the interrupt program endsimmediately.

This interrupt routine thus serves to monitor the input timing t of eachpulse sampling period, i.e. the time t required to receive NC pulses,and signals completion of each sample period (M=0 through M=10, forexample) for the information of the main program.

Before describing the operation in the main routine, the general methodof grouping the sensor pulses into sample periods will be explained tofacilitate understanding of the description of the operation in the mainroutine.

In order to enable the controller unit 202 to accurately calculate thewheel acceleration and deceleration a_(w), it is necessary that thedifference between the pulse intervals of the single or grouped sensorpulses exceeding a given period of time, e.g. 4 ms. In order to obtainthe pulse interval difference exceeding the given period of time, 4 ms,which given period of time will be hereafter referred to as "pulseinterval threshold S", some sensor pulses are ignored so that therecorded input timing t of the sensor pulse groups can satisfy thefollowing formula:

    dT=(C-B)-(B-A)≧S(4 ms.)                             (3)

where A, B and C are the input times of three successive sensor pulsegroups.

The controller unit 202 samples the input timing of three successivesensor pulses to calculate the pulse interval difference dT whileoperating in MODE 1. If the derived pulse interval difference is equalto or greater than the pulse interval threshold S, then sensor pulseswill continue to be sampled in MODE 1. Otherwise, the input timing ofevery other sensor pulse is sampled in MODE 2 and from the sampled inputtiming of the next three sensor pulses sampled, the pulse intervaldifference dT is calculated to again be compared with the pulse intervalthreshold S. If the derived pulse interval difference is equal to orgreater than the pulse interval threshold S, we remain in MODE 2.Otherwise, every four sensor pulses are sampled in MODE 3. The inputtimings of the next three sampled sensor pulses are processed to derivethe pulse interval difference dT. The derived pulse interval differencedT is again compared with the pulse interval threshold S. If the derivedpulse interval difference is equal to or greater than the pulse intervalthreshold S, the MODE remains at 3 and the value N is set to 4. On theother hand, if the derived pulse interval difference dT is less than thepulse interval threshold S, the mode is shifted to MODE 4 to sample theinput timing of every eighth sensor pulse. In this MODE 4, the value Nis set at 8.

Referring to FIG. 14, the main routine serves to periodically derive anupdated wheel acceleration rate value a_(w). In general, this done bysampling larger and larger groups of pulses until the difference betweenthe durations of the groups is large enough to yield an accurate value.In the main routine, the sample flag FL is reset to zero at a block2001. Then the counter value M of the auxiliary counter 233, indicatingthe current sample period of the current a_(w) calculation cycle, isread out at a block 2002 to dictate the subsequent program steps.

Specifically, after the first sample period (M=.0.), the input timing ttemporarily stored in the temporary register 231 corresponding to thesensor pulse number (M=0) is read out and transferred to a memory block240 of RAM at a block 2004, which memory block 240 will be hereafterreferred to as "input timing memory". Then control passes to the block1008 of the main program. When M=2, the corresponding input timing t isread out from the temporary register 231 and transferred to the inputtiming memory 240 at a block 2006. Then, at a block 2008, a pulseinterval Ts between the sensor pulses of M=1 is derived from the twoinput timing values in the input timing memory 240. That is, the pulseinterval of the sensor pulse (M=1) is derived by:

    Ts=t.sub.1 -t.sub.0

where

t₁ is input time of the sensor pulse M1; and

t₀ is input time of the sensor pulse M0.

The derived pulse interval T_(s) of the sensor pulse M1 is then comparedwith a reference value, e.g. 4 ms., at a block 2010. If the pulseinterval T_(s) is shorter than the reference value, 4 ms., controlpasses to a block 2012 wherein the value N and the pulse interval T_(s)are multiplied by 2. The doubled timing value (2T_(s)) is again comparedwith the reference value by returning to the block 2010. The blocks 2010and 2012 constitute a loop which is repeated until the pulse interval(2nT_(s)) exceeds the reference value. When the pulse interval (2nT_(s))exceeds the reference value at the block 2010, a corresponding value ofN (2N) is automatically selected. This N value represents the number ofpulses to be treated as a single pulse with regard to timing.

After setting the value of N and thus deriving the sensor pulse groupsize then the auxiliary counter value NC is set to 1, at a block 2016.The register value N is then checked for a value of 1, at a block 2018.If N=1, then the auxiliary counter value M is set to 3 at a block 2020and otherwise control returns to the main program. When the registervalue N equals 1, the next sensor pulse, which would normally beignored, will instead be treated as the sensor pulse having the sampleperiod number M=3.

In the processing path for the sample period number M=3, thecorresponding input timing is read from the corresponding address of thetemporary register 231 and transferred to the input timing memory 240,at a block 2024. The pulse interval T₂ between the sensor pulses at M=1and M=3 is then calculated at a block 2026. The derived pulse intervalT₂ is written in a storage section of a memory block 242 of RAM 236 fora current pulse interval data, which storage section will be hereafterreferred at as "first pulse interval storage" and which memory block 242will be hereafter referred to as "pulse interval memory". After theblock 2026, control returns to the main program to await the next sensorpulse, i.e. the sensor pulse for sample period number M=4.

When the sensor pulse for M=4 is received, the value t of the temporaryregister 231 is read out and transferred to the input timing memory 240at block 2028. Based on the input timing of the sensor pulses for M=3and M=4, the pulse interval T₃ is calculated at a block 2030. The pulseinterval T₃ derived at the block 2030 is then written into the firstpulse interval storage of the pulse interval memory. At the same time,the pulse interval data T₂ previously stored in the first pulse intervalstorage is transferred to another storage section of the pulse intervalmemory adapted to store previous pulse interval data. This other storagesection will be hereafter referred to as "second pulse intervalstorage". Subsequently, at a block 2032 the contents of the first andsecond storages, i.e. the pulse interval data T₂ and T₃ are read out.Based on the read out pulse interval data T₂ and T₃, a pulse intervaldifference dT is calculated at block 2032 and compared with the pulseinterval threshold S to determine whether or not the pulse intervaldifference dT is large enough for accurate calculation of wheelacceleration or deceleration a_(w). If so, process goes to the block2040 to calculate the wheel acceleration or deceleration according tothe equation (1). The register value N is then set to 1 at the block2044 and thus MODE 1 is selected. In addition sample period number M isreset to -1, and the a_(w) derivation cycle starts again. On the otherhand, if at the block 2032 the pulse interval difference dT is too smallto calculate the wheel acceleration or deceleration a_(w), then thevalue N is multiplied by 2 at a block 2034. Due the updating of thevalue N, the sample mode of the sensor pulses is shifted to the nextmode.

When the block 2034 is performed and thus the sample mode is shifted toMODE 2 with respect to the sensor pulse of M=4', the sensor pulse c₂input following to the sensor pulse of M=4' is ignored. The sensor pulsec₃ following to the ignored sensor pulse c₂ is then taken as the sensorpulse to be sampled as M=3". At this time, the sensor pulse of M=4' istreated as the sensor pulse of M=2" and the sensor pulse of M=2 istreated as the sensor pulse of M=1". Therefore, calculation of theinterval difference dT and discrimination if the derived intervaldifference dT is greater than the pulse interval threshold S in theblock 2032 will be carried out with respect to the sensor pulse c₃ whichwill be treated as the sensor pulse of M=4". The blocks 2032 and 2034are repeated until the interval difference greater than the pulseinterval threshold S is obtained. The procedure taken in each cycle ofrepetition of the blocks 2032 and 2034 is substantially same as that setforth above.

As set forth above, by setting the counter value NC of the auxiliarycounter 233 to 1 at the block 2016, the input timing of the sensor pulsereceived immediately after initially deriving the sample mode at theblocks 2010 and 2012 will be sampled as the first input timing to beused for calculation of the wheel acceleration and deceleration. Thismay be contrasted with the procedure taken in the known art.

FIG. 15 shows the output program for deriving the wheel speed V_(w),wheel acceleration and deceleration a_(w) and slip rate R, selecting theoperational mode, i.e. application mode, hold mode and release mode andoutputting an inlet signal EV and/or an outlet signal AV depending uponthe selected operation mode of the actuator 16.

When the application mode is selected the inlet signal EV goes HIGH andthe outlet signal EV goes HIGH. When the release mode is selected, theinlet signal EV goes LOW and the outlet signal AV also goes LOW. Whenthe selected mode is the hold mode, the inlet signal EV remains HIGHwhile the outlet signal AV goes LOW. These combinations of the inletsignal EV and the outlet signal AV correspond to the actuator supplycurrent levels shown in FIG. 11 and thus actuate the electromagneticvalve to the corresponding positions illustrated in FIGS. 4, 5 and 6.

The output program is stored in the memory block 254 and adapted to beread out periodically, e.g. every 10 ms, to be executed as an interruptprogram.

During execution of the output calculation program, the pulse interval Tis read out from a memory block 241 of RAM which stores the pulseinterval, at a block 5002. As set forth above, since the pulse intervalT is inversely proportional to the wheel rotation speed V_(w), the wheelspeed can be derived by calculating the reciprocal (1/T) of the pulseinterval T. This calculation of the wheel speed V_(w) is performed at ablock 5004 in the output program. After the block 5004, the target wheelspeed V_(i) is calculated at a block 5006. Manner of deriving the targetwheel speed V_(i) has been illustrated in the U.S. Pat. Nos. 4,392,202to Toshiro MATSUDA, issued on July 5, 1983, 4,384,330 also to ToshiroMATSUDA, issued May 17, 1983 and 4,430,714 also to Toshiro MATSUDA,issued on Feb. 7, 1984, which are all assigned to the assignee of thepresent invention. The disclosure of the above-identified three UnitedStates Patents are hereby incorporated by reference for the sake ofdisclosure. As is obvious herefrom, the target wheel speed V_(i) isderived as a function of wheel speed deceleration as actually detected.For instance, the wheel speed V_(w) at which the wheel decelerationa_(w) exceeds the deceleration threshold a_(ref), e.g. -1.2G is taken asone reference point for deriving the target wheel speed V_(i). The wheelspeed at which the wheel deceleration a_(w) also exceeds thedeceleration threshold a_(ref), is taken as the other reference point.In addition, the period of time between the points a and b is measured.Based on the wheel speed V_(w1) and V_(w2) and the measured period P,the deceleration rate dV_(i) is derived from:

    dV.sub.i =(V.sub.w1 -V.sub.w2)/P                           (4)

This target wheel speed V_(i) is used for skid control in the next skidcycle.

It should be appreciated that in the first skid cycle, the target wheelspeed V_(i) cannot be obtained. Therefore, for the first skid cycle, apredetermined fixed value will be used as the target wheel speed V_(i).

At a block 5008 (FIG. 15), the slip rate R is calculated according tothe foregoing formula (2). Subsequently, the operational mode isdetermined on the basis of the wheel acceleration and deceleration a_(w)and the slip rate R, at a block 5010. FIG. 16 shows a table used indetermining or selecting the operational mode of the actuator 16 andwhich is accessed according to the wheel acceleration and decelerationa_(w) and the slip rate R. As can be seen, when the wheel slip rate R isin the range of 0 to 15%, the hold mode is selected when the wheelacceleration and deceleration a_(w) is lower than -1.0G and theapplication mode is selected when the wheel acceleration anddeceleration a_(w) is in the range of -1.0G to 0.6G. On the other hand,when the slip rate R remains above 15%, the release mode is selectedwhen the wheel acceleration and deceleration a_(w) is equal to or lessthan 0.6G, and the hold mode is selected when the wheel acceleration anddeceleration is in a range of 0.6G to 1.5G. When the wheel accelerationand deceleration a_(w) is equal to or greater than 1.5G, the applicationmode is selected regardless of the slip rate.

According to the operational mode selected at the block 5010, the signallevels of the inlet signal EV and the outlet signal AV are determined sothat the combination of the signal levels corresponds to the selectedoperation mode of the actuator 16. The determined combination of theinlet signal EV and the outlet signal AV are output to the actuator 16to control the electromagnetic valve.

It should be appreciated that, although the execution timing of theoutput calculation program has been specified to be about 10 ms in theforegoing disclosure, the timing is not necessarily fixed to thementioned timing and may be selectable from the approximate range of 1ms to 20 ms. The execution timing of the output program is fundamentallyto be determined in accordance with the response characteristics of theactuator.

The output program and the main program are executed on a timing-sharingbasis. As shown in FIG. 17, the output program and the main program areexecuted within a fixed period of time, e.g. every 10 ms. Therefore, thewheel speed V_(w) can be derived every 10 ms, as shown in FIG. 17,during execution of the output program. During execution of the outputprogram, the slip rate R can be derived every 10 ms. On the other hand,as set forth with respect to the main routine of FIG. 14 and illustratedin FIG. 18, the wheel acceleration a_(w) is not always derived every 10ms. Specifically, the wheel acceleration a_(w) is calculated only afterthe difference between the pulse intervals of the signals or groupedsensor pulses becomes larger than a given period of time. For example,when sample mode MODE 4 is selected, the acceleration and decelerationof the wheel will be derived every eight cycles of main programexecution, as shown in FIG. 19.

As set forth with respect to FIG. 16, the operational mode of theactuator 16 is determined relative to the wheel acceleration anddeceleration a_(w) and the slip rate R, at the block 5010 of the outputprogram. As this block for determining the operational mode of theactuator is part of the output program, the operational mode may changeevery 10 ms. Since the wheel speed V_(w) and the slip rate R is derivedas part of the main program, a fresh slip rate value is obtained eachtime the block 5010 is executed. However, with regard to the wheelacceleration data, new values are not obtained as frequently and olddata must necessarily be used until wheel acceleration is newlycalculated. As the operational mode is determined in terms of the wheelacceleration and deceleration a_(w) and the slip rate R, it is possiblethat one or more operation modes which constitute part of each cycle ofanti-skid control may be skipped due to this delay in the calculation offresh wheel acceleration values.

For instance, in the normal state in which the calculation of the wheelacceleration data is not so significantly delayed as to affect selectionof the operational mode, the Lissajous figure relating the wheelacceleration and the slip rate is almost a perfect circle, as shown inFIG. 20. On the other hand, when the delay is significant, thecorresponding Lissajous figure may become sufficiently oblate to skipone of the modes, as shown in FIG. 21. In FIGS. 20 and 21, a₁ and b₁represent the hold mode, a₀ represents application mode and b₂represents release mode.

FIG. 22 charts selection of the operational mode relative to variationsin the wheel speed and the wheel acceleration. FIG. 22(c) corresponds tothe normal cycle of skid control illustrated in FIG. 20 and FIG. 22(d)corresponds to a cycle in which one or more operational modes areskipped. In the example shown in FIG. 22, b₁ (holding mode) is skippedso that application mode a₀ follows releae mode b₂ directly. In thiscase, since the brake system enters application mode immediately afterthe wheel speed V_(w) reaches the optimal wheel speed (shown indashed-dotted lines in FIG. 22a) derived from an assumed optimal sliprate and assumed optimal vehicle speed during braking, the wheel speedwill very quickly drop again below the optimal leve, triggering asubsequent hold mode which only serves to drive the wheel speed lower.Accordingly, the slip rate R increases beyond an allowable limit, whichmay result in a skid despite the anti-skid control operation. Inaddition, it can be expected that due to the increase in the slip ratebeyond the allowable limit, the braking distance will be dangerouslyprolonged.

Therefore, in order to prevent the anti-skid control system fromskipping one or more operational modes at the block 5010 of the outputprogram, the control mode select routine of the block 5010 includessteps for checking the sequence of operational modes and correcting thedetermined mode so that all of the necessary operational modes areperformed in order in each skid cycle.

FIG. 23 is a flowchart of one example of the control mode select routineof the block 5010 of FIG. 15. This example is designed to prevent thecircumstances shown in FIG. 22(d), i.e. release mode being followeddirectly by application mode, this case being particularly liable toresult in noticeable and possibly dangerous malfunction of the brakesystem. It is, of course, also possible for the control mode selectroutine to check all possible mode changes, or only selected othermodes.

In the control mode select routine, the wheel acceleration a_(w) and theslip rate R are read out at a first block 5010-1. The table is referredto according to these values at a block 5010-2 in order to determine theoperational mode of the actuator 16.

In subsequent block 5010-3, a release mode-indicative flag FLb₂ ischecked to see if it is set, indicating that the system is currently ina release mode. If the flag FLb₂ is not set, then control passes to ablock 5010-5, described below. Therefore, in this case, the operationalmode of the actuator 16 is output as derived from the look-up tablewithout change.

On the other hand, when the flag FLb₂ is set when checked at the block5010-3, then the determined operational mode is checked to see if it isthe application mode, at a block 5010-4. When the determined operationalmode is not the application mode as checked at the block 5010-4, thenthe determined operational mode is again checked to see if it is therelease mode at a block 5010-5. If the release mode is detected at theblock 5010-5, then control returns to the output program after the flagFLb₂ is set in block 5010-6. Thus, the determined release mode isactuated as dictated by the look-up table. On the other hand, if thedetermined operational mode is not the release mode when checked at theblock 5010-5, then the flag FLb₂ is reset at a block 5010-7. Thereaftercontrol returns to the output program.

When the determined operational mode when checked at the block 5010-6 isthe application mode (a₀), then the operational mode is changed to theholding mode (a₁) at a block 5010-8. Thereafter, the flag FLb₂ is resetat the block 5010-7. Therefore, the operational mode becomes holdingmode.

As set forth above, according to the present invention, the holding modeis always carried out subsequent to the release mode and thus skippingof the holding mode between the release mode and the application mode issatisfactorily avoided. However, the shown embodiment uses a flag toidentify the preceding operational mode, this may be done in variousways. In addition, although the shown embodiment prevents the controlsystem from skipping the hold mode between the release mode and theapplication mode, it is possible as stated previously to provide a morecomprehensive program for avoiding skipping of any mode in each cycle ofskid control operation.

FIG. 24 shows another embodiment of the controller unit 202 in thepreferred embodiment of the anti-skid control system according to thepresent invention. In practice, the circuit shown in FIG. 24 performsthe same procedure in controlling the actuator 16 and each block of thecircuit performs operation substantially corresponding to that performedby the foregoing computer flowchart.

In FIG. 24, the wheel speed sensor 10 is connected to a shaping circuit260 provided in the controller unit 202. The shaping circuit 260produces the rectangular sensor pulses having a pulse interval inverselyproportional to the wheel speed V_(w). The sensor pulse output from theshaping circuit 260 is fed to a pulse pre-scaler 262 which counts thesensor pulses to produce a sample command for sampling input timing whenthe counter value reaches a predetermined value. The predetermined valueto be compared with the counter value in the pulse pre-scaler 262 isdetermined such that the intervals between the pairs of three successivesample commands will be sufficiently different to allow calculation ofthe wheel acceleration and deceleration rate.

The sample command is fed to a flag generator 264. The flag generator264 is responsive to the sample command to produce a flag signal. Theflag signal of the flag generator 264 is fed to a flag counter 266 whichis adapted to count the flag signals and output a counter signal havinga value representative of its counter value.

At the same time, the sample command of the pulse pre-scaler 262 is fedto a latching circuit 268 which is adapted to latch the signal value ofa clock counter signal from a clock counter 267 counting the clock pulseoutput by a clock generator 11. The latched value of the clock countersignal is representative of the input timing of the sensor pulse whichactivates the pulse pre-scaler 262 to produce the sample command. Thelatching circuit 268 sends the input timing indicative signal having avalue corresponding to the latched clock counter signal value, to amemory controller 274. The memory controller 274 is responsive to amemory command input from an interrupt processing circuit 272 which inturn is responsive to the flag counter signal to issue a memory commandwhich activates the memory controller 274 to transfer the input timingindicative signal from the latching circuit 268 to a memory area 276.The memory 276 sends the stored input timing indicative signal to asample controller 270 whenever the input timing signal valuecorresponding to the latched value of the latching circuit 268 iswritten therein. The sample controller 270 performs operationssubstantially corresponding to that performed in the blocks 2008, 2010,2012, 2032 and 2034 in FIG. 15, i.e. it determines number of sensorpulses in each group to be ignored. The sample controller 270 outputs apulse number indicative signal to the pulse pre-scaler 262, which pulsenumber indicative signal has a value approximating the predeterminedvalue to be compared with the counter value in the pulse pre-scaler 262.

The memory 276 also feeds the stored input timing indicative signal to awheel acceleration and deceleration calculation circuit 278 and a pulseinterval calculation circuit 280. The wheel acceleration anddeceleration calculation circuit 278 first calculates a pulse intervaldifference between pairs of three successively sampled sensor pulses.The obtained pulse interval difference is compared with a referencevalue so as to distinguish if the pulse interval difference is greatenough to allow calculation of the wheel acceleration and decelerationa_(w). If the obtained pulse interval difference is greater than thereference value, then the wheel acceleration and decelerationcalculation circuit 278 performs calculation of the wheel accelerationand deceleration according to the foregoing formula (1). If the obtainedpulse interval difference is smaller than the reference value, the wheelacceleration and deceleration calculation circuit 278 shifts theoperational mode thereof in a manner substantially corresponding to theprocedure disclosed set forth above, so as to achieve a pulse intervaldifference large enough to permit the wheel acceleration anddeceleration calculation. On the other hand, the pulse intervalcalculation circuit 280 performs calculations to obtain the pulseinterval between the current pulse and the immediate preceding pulse andsends a pulse interval indicative signal to a memory 282. The memory 282sends a stored pulse interval indicative signal to a wheel speedcalculation circuit 284 which is associated with a 10 ms timer 292. The10 ms timer 292 produces a timer signal every 10 ms to activate thewheel speed calculation circuit 284. The wheel speed calculation circuit284 is responsive to the timer signal to perform calculation of thewheel speed V_(w) by calculating the reciprocal value of the pulseinterval indicative signal from the memory 282. The wheel speedcalculation circuit 284 thus produces a wheel speed indicative signal tobe fed to a target wheel speed calculation circuit 288 and to a sliprate calculation circuit 290 which is also associated with the 10 mstimer to be activated by the timer signal every 10 ms.

The target wheel speed calculation circuit 288 is adapted to detect thewheel speed V_(w) at which the wheel acceleration and deceleration a_(w)calculated by the wheel acceleration and deceleration calculatingcircuit 278 exceeds than a predetermined deceleration rate -b. Thetarget wheel speed calculation circuit 288 measures the interval betweentimes at which the wheel deceleration exceeds the predetermineddeceleration value. Based on the wheel speed at the foregoing times andthe measured period of time, the target wheel speed calculation circuit288 derives a decelerating ratio of the wheel speed to produce a targetwheel speed indicative signal. The target wheel indicative signal of thetarget wheel speed calculation circuit 288 and the wheel speedindicative signal from the wheel speed calculation circuit 284 are fedto a slip rate calculation circuit 290.

The slip rate calculation circuit 290 is also responsive to the timersignal from the 10 ms timer 292 to perform calculation of the slip rateR based on the wheel speed indicative signal from the wheel speedcalculation circuit 284 and the target wheel speed calculation circuit288, in accordance with the formula (2).

The slip rate calculation circuit 290 and the wheel acceleration anddeceleration calculation circuit 278 are connected to an output unit 294to feed the acceleration and deceleration indicative signal and the sliprate control signal thereto. The output unit 294 determines theoperation mode of the actuator 16 based on the wheel acceleration anddeceleration indicative signal value and the slip rate indicative signalvalue according to the table of FIG. 16. The output unit 294 thusproduces the inlet and outlet signals EV and AV with a combination ofsignal levels corresponding to the selected operation mode of theactuator.

On the other hand, the wheel speed calculation circuit 284 is alsoconnected to the flag counter 266 to feed a decrementing signal wheneverthe calculation of the wheel speed is completed and so decrement thecounter value of the flag counter by 1. The flag counter 266 is alsoconnected to a comparator 295 which is adapted to compare the countervalue of the flag counter with a reference value, e.g. 2. When thecounter value of the flag counter 266 is greater than or equal to thereference value, the comparator 295 outputs a comparator signal to anoverflow detector 296. The overflow detector 296 is responsive to thecomparator signal to feed a sample mode shifting command to be fed tothe pulse pre-scaler 262 to shift the sample mode to increase the numberof the sensor pulses in each sample group.

On the other hand, the clock counter 267 is connected to an overflowflag generator 297 which detects when the counter value reaches the fullcount of the clock counter to produce an overflow flag signal. Theoverflow flag signal of the overflow flag generator 297 is fed to anoverflow flag counter 298 which is adapted to count the overflow flagsignals and send an overflow counter value indicative signal to ajudgment circuit 299. The judgment circuit 299 compares the overflowcounter indicative signal value with a reference value e.g. 2. Thejudgment circuit 299 produces a reset signal when the overflow counterindicative signal value is equal to or greater than the reference value.The reset signal resets the wheel acceleration and decelerationcalculation circuit 278 and the wheel speed calculation circuit 284 tozero. On the other hand, the overflow flag counter is connected to thewheel speed calculation circuit 284 and is responsive to thedecrementing signal output from the wheel speed calculation circuit asset forth above to be reset in response to the decrementing signal.

FIG. 25 shows the output unit 294 of the controller unit 202 of FIG. 24.The output unit 294 includes a mode selector circuit 294-1 including amemory 294-2 storing the table of the control modes accessible in termsof the wheel acceleration and the slip rate, and an error signalgenerator 294-3 associated with the mode selector circuit 294-1 formonitoring the control mode determined by the mode selector circuit.

In order to determine the control mode, the mode selector circuit 294-1is connected to the slip rate calculation circuit 290 to receive asignal indicative of slip rate. Also, the mode selector circuit 294-1 isconnected to the wheel acceleration calculation circuit 278 to receivethe wheel acceleration indicative signal. Based on the slip rateindicative signal value and the wheel acceleration indicative signalvalues, the mode selector circuit 294-1 derives a control signalindicative of the control mode determined by table loop-up. The controlsignal derived by the mode selector 294-1 comprises the inlet signal EVand the outlet signal AV. The combination of signal levels of the inletsignal EV and the outlet signal AV vary according to the selectedoperation mode of the actuator, which combinations are summarized in thefollowing table:

    ______________________________________                                        Application        Hold    Release                                            ______________________________________                                        EV     HIGH            HIGH    LOW                                            AV     HIGH            LOW     LOW                                            ______________________________________                                    

Therefore, the mode selector circuit 294-1 has an inlet signal outputterminal 294-4 which will hereafter be referred to as "EV terminal", andan outlet signal output terminal 294-5 which hereafter be referred to as"AV terminal". The AV terminal 294-5 is connected to one input terminalof an AND gate 294-6. The other input terminal of the AND gate 294-6 isconnected to the error signal generator 294-3.

The error signal generator 294-3 is designed to detect when either theapplication mode or the release mode is selected by the mode selectorcircuit 294-1. The error signal generator 294-3 normally sends aHIGH-level output to the AND gate 294-6. In this case when the outletsignal AV is HIGH, the AND gate output goes HIGH, and when the outletsignal AV is low, the output of the AND gate 294-6 goes LOW. The errorsignal generator 294-3 detects when the application mode is selectedimmediately after the releasing mode and in such cases, switches itsoutput level to LOW. In this case, the output level at the AV terminal294-5 is HIGH as the application mode is selected. Due to the LOW-levelinput from the error signal generator 294-3, the output level of the ANDgate 294-6 goes LOW. Therefore, the hold mode is performed. Thereafter,the error signal generator 294-3 returns to normal state, sending aHIGH-level signal to the AND gate 294-6.

FIG. 26 shows a modification of the foregoing embodiment of FIG. 25. Inthis modification, in the table in which the operational modes arestored for later reference in terms of the wheel acceleration a_(w) andthe slip rate R, the operational modes are represented in the form of3-bit binary numbers, as shown in FIG. 27. The mode selector circuit294-1' therefore includes a digital memory 294-2' storing theoperational mode-indicative binary numbers. The mode selector circuit294-1' is also provided with another memory 294-11 identifying whichmode was selected in the previous cycle, and a calculation circuit294-12 which compares the binary number stored in the memory 294-11 withthe binary number selected from memory 294-2'. The binary numberindicative of the previous operation mode will be hereafter referred toas "old mode data N_(n-1) " and the binary number indicative of thecurrent operational mode will be referred hereafter as "new mode dataN_(n) ". The calculation circuit 294-12 produces a signal indicative ofthe difference between the old operational mode and the new operationmode, i.e.:

    dN=|N.sub.n -N.sub.n-1 |

The output signal of the calculation circuit 294-12 is fed to an errorsignal generator 294-3'. The error signal generator 294-3' has twooutput terminals 294-13 and 294-14 respectively connected to an AND gate294-15 and an OR gate 294-16. The AND gate 294-15 is also connected tothe EV terminal 294-4'. The OR gate 294-16 is also connected to the AVterminal 294-5 of the mode selector circuit 294-1'.

In general, the error signal generator 294-3' is adapted to detectabnormal switching of the operational mode which would otherwise skipone of the application mode and release mode. For instance, the errorsignal generator 294-3' detects switching of operational mode from thehold mode (001) to the hold mode (011), skipping the application mode(000).

In the normal state, the error signal generator feeds a HIGH-leveloutput to the AND gate 294-15 and a LOW-level output to the OR gate294-16. Upon detecting skipping of either the application mode or therelease mode, the error signal generator adjusts its combination ofoutputs according to the following pattern:

    ______________________________________                                                        ABNORMAL                                                                      (APPLI-     ABNORMAL                                                  NORMAL  CATION)     (RELEASING)                                       ______________________________________                                        AND gate input                                                                          HIGH      HIGH        LOW                                           OR gate input                                                                           LOW       HIGH        LOW                                           ______________________________________                                    

Therefore, in case where the application mode would be skipped byselecting the hold mode (001) immediately after the hold mode (011), theoutput of the error signal generator 294-3' to the OR gate 294-16 goesHIGH. At this time, although the AV output in the hold mode is LOW, theOR gate output goes HIGH. As a result, the AND gate output, i.e. the EVsignal and the OR gate output, i.e. the AV signal both go HIGH,actuating the application mode.

On the other hand, when the release mode is to be skipped due to changeof the operational mode from hold mode (011) to the hold mode (001), theoutput of the error signal generator 294-3' to the AND gate 294-15 goesLOW. Although the EV output from the EV terminal 294-4' is HIGH in thehold mode, the output of the AND gate goes LOW. Therefore, both of theoutputs of the AND gate 294-15 and the OR gate 294-16 i.e. the EV and AVsignals go LOW to perform the fluid control operation under the releasemode.

As can be appreciated from the table of FIG. 27, when the operationalmodes are selected in the normal state, the difference between theselected mode-indicative signals will be 001 or 011 and on the otherhand, when the mode selection is abnormal in that it would skip eitherthe application mode or the release mode, the difference would be 010.Thereafore, by detecting when the difference is 010, the error isdetected. In addition, by checking the binary number of either the oldor the new operational mode, which of the application mode or therelease mode should be performed can be detected.

Therefore, according to the invention, skipping of operational modes canbe effectively prevented by modifying the selected operational mode tothe mode skipped. This fulfills and satisfies all of the object andadvantages sought for the invention.

What is claimed is:
 1. An anti-skid brake control system for anautomotive hydraulic brake system comprising:a hydraulic brake circuitincluding a wheel cylinder for applying braking force to a vehiclewheel; a pressure control valve disposed within said hydraulic circuitand operative to increase fluid pressure in said wheel cylinder in afirst position thereof, to decrease the fluid pressure in said wheelcylinder in a second position thereof and to hold the fluid pressure insaid wheel cylinder constant in a third position thereof; a wheel speedsensor means for detecting the rotational speed of said vehicle wheeland producing sensor signal pulses separated by intervals, saidintervals representative of detected wheel speed; a first means fordetecting wheel acceleration and producing a first signal indicative ofthe detected wheel acceleration; a second means for detecting the sliprate of the vehicle wheel with respect to the ground and producing asecond signal indicative of the detected slip rate; a third means forproducing a control signal which actuates said pressure control valve ina predetermined sequence of positions of said first, second and thirdpositions so as to adjust the wheel rotational speed toward an optimalrelationship with vehicle speed, said third means generating saidcontrol signal based on a predetermined relationship between values ofsaid first and second signals; and a fourth means, associated with saidthird means and responsive to said control signal for determining anintended sequence of operations of said pressure control valve in saidfirst, second and third positions and for modifying said control signalwhen said control signal would otherwise control said pressure controlvalve to operate outside said predetermined sequence of positions toassure that said pressure control valve always operates in saidpredetermined sequence of positions.
 2. The anti-skid brake controlsystem as set forth in claim 1, wherein said third means selects atleast one of first, second, third and fourth modes in a predeterminedpattern according to said predetermined sequence of positions wherein,in said first mode said pressure control valve is operated to said firstposition, in said second mode, said pressure control valve is operatedto said third position at a first constant value of fluid pressure, insaid third mode, said pressure control valve is operated to said secondposition and in said fourth mode, said pressure control valve isoperated to said third position at a second constant value of fluidpressure.
 3. The anti-skid brake control system as set forth in claim 2,wherein said fourth means detects one of said first and third modesselected by said third means after skipping one of said second andfourth modes and modifies the control signal to control said pressurecontrol valve to one of said skipped second and fourth modes.
 4. Theanti-skid brake control system as set forth in claim 2, wherein saidfourth means detects one of said second and fourth modes selected bysaid third means after skipping one of said first and third modes andmodifies the control signal to control said pressure control valve toone of said skipped first and third modes.
 5. The anti-skid brakecontrol system as set forth in claim 3, wherein said control modes arestored in a look-up table and said third means selects one of saidcontrol modes by means of said look-up table in terms of said first andsecond signal values.
 6. The anti-skid brake control system as set forthin claim 4, wherein said control modes are stored in a look-up table andsaid third means selects one of said control modes by means of saidlook-up table in terms of said first and second signal values.
 7. Theanti-skid brake control system as set forth in claim 2, which furthercomprises a timer means for producing a timer signal and a fifth meansfor sampling values of said timer signal for determining input timing.8. The anti-skid brake control system as set forth in claim 7, whereinsaid fifth means samples said timer signal at a given number of sensorsignal pulses, each given number of sensor signal pulses forming agroup, said number being adjusted so that a difference of the timedurations of each group is greater than a predetermined value.
 9. Theanti-skid brake control system as set forth in claim 8, wherein saidthird means derives a value of wheel acceleration or deceleration fromthe difference between the time durations of each three successivegroups of sensor signal pulses.
 10. The anti-skid brake control systemas set forth in claim 9, wherein said third means derives a value ofslip rate based on a wheel speed derived from the time interval betweensuccessive sensor signal pulses and an assumed vehicle speed based onthe wheel speed at the beginning of said first mode.
 11. The anti-skidbrake control system as set forth in claim 10, wherein said fourth meansdetects one of said first and third modes selected by said third meansafter skipping one of said second and fourth modes and modifies thecontrol signal to control the pressure control valve to one of saidskipped second and fourth modes.
 12. The anti-skid brake control systemas set forth in claim 10, wherein said fourth means detects one of saidsecond and fourth modes selected by said third means after skipping oneof said first and third modes and modifies the control signal to controlthe pressure control valve to one of said skipped first and third modes.13. The anti-skid brake control system as set forth in claim 11, whereinsaid third means selects one of said control modes by means of a look-uptable, which table stores binary numbers respectively representative ofone of said control modes.
 14. The anti-skid brake control system as setforth in claim 12, wherein said third means selects one of said controlmodes by means of a look-up table, which table stores binary numbersrespectively representative of one of said control modes.
 15. Theanti-skid brake control system as set forth in claim 13, wherein saidbinary numbers correspond to respective control modes so that adifference of the previously selected binary number and the currentlyselected binary number has a specific value when the currently selectedcontrol modes are not within said predetermined pattern.
 16. Theanti-skid brake control system as set forth in claim 14, wherein saidbinary numbers correspond to respective control modes so that adifference of the previously selected binary number and the currentlyselected binary number has a specific value when the currently selectedcontrol modes are not within said predetermined pattern.
 17. Ananti-skid method for controlling an automotive brake system including ahydraulic brake circuit with a wheel cylinder and a pressure controlvalve associated with said wheel cylinder for controlling fluid pressureto be applied to the wheel cylinder, said pressure control valveoperative to increase the fluid pressure in said wheel cylinder in afirst position thereof, to decrease the fluid pressure in said wheelcylinder in a second position thereof and to hold the fluid pressure ata constant value in a third position thereof, the method comprising thesteps of:detecting a braking condition including a wheel acceleration ordeceleration to produce a braking condition indicative signal; selectinga plurality of control modes each corresponding to one of said first,second and third valve positions according to a predetermined operationpattern based on said braking condition indicative signal, to produce acontrol signal representative of the selected control modes foroperating said pressure control valve to the corresponding valveposition; and detecting the selected control modes and modifying thecontrol signal when the detected control modes are not within saidpredetermined operation pattern, said pressure control valve responsiveto said modified control signal to assume valve positions in accordancewith said predetermined operation pattern.
 18. The method as set forthin claim 17, which further comprising a step of storing binary numbersrepresentative of said control modes, one of said binary numbers beingidentified by each value of said braking condition indicative signal.19. The method as set forth in claim 17, in which said wheelacceleration or deceleration is a first factor in said braking conditionindicative signal and a slip rate as a second factor in said brakingcondition indicative signal.
 20. The method as set forth in claim 19,wherein in selecting the control modes, at least one of first, second,third and fourth modes is selected according to the predeterminedoperation pattern depending upon said braking condition indicativesignal, in said first mode, said pressure control valve being operatedto said first position, in said second mode, said pressure control valvebeing operated to said third position at a first constant value of fluidpressure, in said third mode, said pressure control valve being operatedto said second position and in said fourth mode, said pressure controlvalve being operated at said third position at a second constant valueof fluid pressure.
 21. The method as set forth in claim 20, wherein, atsaid step for detecting the control modes, one of said second and fourthmodes selected by skipping one of said first and third modes is detectedso as to modify the control signal to control said pressure controlvalve to one of said skipped first and third modes.
 22. The method asset forth in claim 20, wherein, at said step of detecting the controlmodes, one of said first and third modes selected by skipping one ofsaid second and fourth modes is detected so as to modify the controlsignal to control said pressure control valve to one of said skippedsecond and fourth modes.